Back pain affects millions of people and is a common cause of disability for the middle-aged working population. A frequent cause of back pain is rupture or degeneration of intervertebral discs. Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae, and cushion vertebral bodies. An intervertebral disc includes the annulus fibrosus, a structure that surrounds and confines an inner component, the nucleus pulposus. The annulus fibrosis is composed of a ring of collagen fibers and fibrocartilage embedded in a generally amorphous base substance. The nucleus pulposus is comprised of a mucoid material containing mainly glycoproteins and some collagen. In a healthy, undamaged spine, the annulus fibrosus prevents the nucleus pulposus from protruding outside the disc space and also resists torsional and bending forces applied to the disc.
Intervertebral discs may be displaced or damaged due to disease or aging. Disruption of the annulus fibrosus can allow the nucleus pulposus to protrude into the vertebral canal or intervertebral foramen, a condition known as a herniated or slipped disc. A rupture in the annulus fibrosis can allow the escape of nucleus pulposus components. The extruded nucleus pulposus may press on a spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Furthermore, as a disc dehydrates and hardens due to age or disease, the disc space height will be reduced, leading to instability of the spine, decreased mobility and pain. Moreover, excessive movement of the spinal segments caused by the disc space height reduction could weaken the annulus fibrosus and, in certain cases, tear it.
Common methods of providing relief for damaged intervertebral discs include surgical removal of all or a portion of the intervertebral disc, followed by fusion of the adjacent vertebrae. Although fusion can eliminate the above symptoms, the restricted motion of the fused segment increases the range of motion required of the adjoining intervertebral discs and could enhance their degeneration.
To overcome the problems with fusion, replacing the entire intervertebral disc with an artificial, articulating disc is believed to be a viable medical alternative to fusion. Many of these articulating invertebral disc devices utilize multicomponent biocompatible polymeric and metallic materials in an attempt to simulate the normal, healthy intervertebral disc motion.
Several obstacles have been addressed in the design and development of prosthetic, articulating intervertebral disc spacers. For example, the disc prosthesis must have immediate and long-term fixation to bone. Immediate fixation may be accomplished with screws, staples, or “teeth” which are integral to the implant. While these techniques may offer long-term stability, other options include porous or macrotexture surfaces which allow for bone in growth.
However, one need that remains unaddressed is the prevention of undesirable bone growth or scar tissue growth on the articulating surfaces or into the articulating spaces of the prosthetic disc implant. Such scar tissue and unwanted bone growth can undermine the effectiveness of the entire replacement disc by inhibiting or preventing the articulation of the prosthetic disc implant in a manner which promotes normal healthy intervertebral disc motion.